In a VCSEL (Vercial Cavity Surface Emitting Laser) device, it is important to suppress oscillations in a high-order side mode in terms of its application. Therefore, various attempts have been made to achieve this feature. Among them, it is particularly effective to make a reflectance difference between the central part and the peripheral part of an emitting region to suppress the oscillations in the high-order side mode. In this regard, various methods have been disclosed.
For example, Patent Document 1 discloses a surface-emitting laser device in which a layer structure made of a semiconductor material and having a luminous layer arranged between an upper reflecting mirror layer structure and a lower reflecting mirror layer structure is formed on a substrate. An upper electrode having a circular shape in a plan view is formed above the upper reflecting mirror layer structure. An opening part is formed inside the upper electrode and a transparent layer partially covering the front surface of the opening part is formed with respect to the oscillation wavelength of oscillation laser light. Here, a circular SiN film whose optical thickness is an odd number multiple of λ/4 is formed on an emitting surface, thereby reducing a reflectance in the peripheral part of an emitting region.
Further, Patent Document 2 discloses a surface-emitting semiconductor laser having a laser structure in which a first multilayer film reflecting mirror, an active layer having a luminous central region, a second multilayer film reflecting mirror, and a side mode control layer are laminated on a substrate in this order.
In the laser structure, one of the first multilayer film reflecting mirror and the second multilayer film reflecting mirror has a quadrate current injection region in which the intersection point of diagonal lines corresponds to the luminous central region. The second multilayer film reflecting mirror has a light emitting port provided in a region corresponding to one of the diagonals in the current injection region and has a pair of groove parts provided on both sides of the light emitting port. The side mode control layer is provided corresponding to the light emitting port and structured such that the reflectance of a peripheral region other than a central region corresponding to the luminous central region in the light emitting port is lower than that of the central region. Here, as the side mode control layer, a structure in which plural types of dielectric films are alternately laminated with each other to make a reflectance difference is disclosed as well as a structure using one type of dielectric film (see, for example, a fifth embodiment of Patent Document 2). Moreover, the second multilayer film reflecting mirror in the peripheral region of the light emitting port is removed by etching to reduce the reflectance of the peripheral region (see, for example, a fourth embodiment of Patent Document 2).
Further, Patent Document 3 discloses a surface-emitting laser device having a layer structure in which a lower reflecting mirror, an active layer, and an upper reflecting mirror are laminated in this order, and has a current confinement layer provided inside the lower reflecting mirror or the upper reflecting mirror. This surface-emitting laser device has a semiconductor layer provided on the upper reflecting mirror and having a first region showing a first reflectance with respect to oscillation laser light and a second region showing a second reflectance with respect to the oscillation laser light inside the boundary surface of a current confinement region defined at least by the current confinement layer. Here, the thickness of a GaAs layer at the output surface is made different between a central part and a peripheral part by about λ/4 as an optical thickness, thereby reducing the reflectance of the peripheral part.
However, the surface-emitting semiconductor laser device disclosed in Patent Document 1 has a limit in the effect of suppressing the oscillations in the high-order side mode.
Further, in the surface-emitting semiconductor laser disclosed in Patent Document 2, the structure, in which the plural types of dielectric films are alternately laminated to form the side mode control layer, increases the number of processes of manufacturing the surface-emitting semiconductor layer, which is not desirable in terms of mass production and manufacturing cost.
Furthermore, in the surface-emitting semiconductor laser disclosed in Patent Document 2, when the second multilayer reflecting mirror in the peripheral region of the light emitting port is removed by etching, the multilayer reflecting mirror has a smaller reflectance than a dielectric per one pair. Therefore, in order to obtain a large reflectance difference, it is necessary to remove many layers. At this time, if the layers are removed by wet etching, it is presumed that the cross sections of the layers are inclined and the reflectance difference is likely to become unstable at a boundary between a high reflectance region and a low reflectance region.
Further, in the surface-emitting laser device disclosed in Patent Document 3, when the GaAs layer is increased to have an optical thickness of about λ/4 so as to make a large reflectance difference, an inconvenience such as a reduction in luminous efficiency due to an increased absorption loss occurs.    Patent Document 1: JP-A-2001-156395    Patent Document 2: JP-A-2007-201398    Patent Document 3: JP-A-2003-115634